The plastics industry is one of the largest consumers of organic and inorganic fillers. Inorganic fillers such as calcium carbonate, talc, mica and the like are well known, as well as organic fillers such as wood flour, chaff and the like, fibrous materials such as asbestos and glass fiber, as well as graphite, cokes, blown asphalt, activated carbon, magnesium hydroxide, aluminum hydroxide and the like. All of these additives have high specific gravities and their ability to improve physical properties of the composition is limited.
As an alternative to particulate fillers, thermoplastic materials can also be formed with fibrous materials to overcome those deficiencies. Fiber-reinforced composite materials based on thermoplastic materials are being increasingly used in many areas of technology in place of metallic materials as they promise a substantial reduction in weight, with mechanical characteristics which are otherwise comparable in many respects. For that purpose, besides the thermoplastic matrix, these composite materials include a fibrous component which has a considerable influence on mechanical characteristics, in particular tensile and flexural strength as well as impact toughness of the composite material. Fibrous components used are (i) fibers of inorganic materials such as glass, carbon and boron, (ii) metallic fibers, for example of steel, aluminium and tungsten, (iii) synthetic organic fibers, for example of aromatic polyamides, polyvinyl alcohols, polyesters, polyacrylates and polyvinyl chloride, or (iv) fibers of natural origin, for example hemp and flax.
The use of glass fiber-reinforced thermoplastic materials has of particular significance. In FIG. 1, a prior art process for the incorporation of glass fibers into a plastic resin, such as polypropylene, is illustrated. The polypropylene 10 is initially combined at a suitable temperature and pressure with the glass fibers 12 and other additives 14, as desired. The polypropylene 10, glass fibers 12 and additives 14 are mixed to form the composite material 16. This composite material 16 can be subsequently extruded at 18 for use in an injection molding process 20 to form a final molded product 22 having properties provided by the combination of the polypropylene 10 and glass fibers 12, along with any additional desired properties provided by the additives 14.
However, the production of glass fibers requires the use of considerable amounts of energy and the basic materials are not biological in origin so that the sustainability of the production process is open to criticism from ecological points of view. Furthermore, the disposal of glass fiber-reinforced thermoplastic materials is made difficult as even upon thermal decomposition of the material, considerable amounts of residues are left, which generally can only be taken to a disposal site. Finally glass fibers involve a high level of abrasiveness so that processing the materials in the context of usual processing methods for thermoplastic materials encounters difficulties.
Because of the above-mentioned disadvantages but also generally to improve the material properties therefore at the present time there is an intensive search for possible ways of replacing the glass fibers which dominate in many technical uses, as a reinforcing component. Organic fibrous materials of natural origin, such as plant materials appear to be particularly attractive in this connection because of their lower density and the reduction in weight that this entails in the composite material as well as sustainability and easier disposal.
The potential of using natural or plant fibers in plastic applications as a substitute for synthetic fibers such as glass, carbon, nylon, polyester, etc. has been recognized. For example, Kolla et al. U.S. Pat. No. 6,133,348, which is hereby expressly incorporated by reference herein, describes flax shives reinforced thermoplastic compositions and a method for reinforcing thermoplastic resins. The invention disclosed in Kolla provides a use for flax shives or particles in the thermoplastic compositions, which is the portion left over after processing plant materials to separate plant fibers (bast fibers) from the shives. The shives are the core tissue fibers which remain after the bast fibers are removed from the flax stem via the mechanical separation process disclosed in Leduc et al. U.S. Pat. No. 5,906,030, or other mechanical separation processes involving the hammering or bending of the natural plant fibers. These core tissue fibers include the cellulose, hemi-cellulose and lignin components of the flax fiber, along with a smaller portion of the woody bast fibers that remain on the shives, giving the shives a fiber purity of approximately eighty percent, at maximum.
It will be noted however that the use of natural fibrous materials as a fiber-reinforcing component can be confronted with worse mechanical characteristics in the resulting composite materials, in comparison with fiber-reinforced composite materials with glass fiber constituents. Furthermore natural fibers such as flax, hemp or also wood particles are of a fluctuating composition: individual batches of the material differ depending on the respective cultivation area, cultivation period, storage and possibly preliminary treatment. That means however that the mechanical characteristics of the fiber-reinforced thermoplastic materials to be produced also vary, which makes technical use thereof more difficult. The material can further change in form and appearance by virtue of progressing degradation processes. Finally, the constituent components of the various natural fibers can themselves create issues when the fibers are utilized in this manner. In particular, the hemi-cellulose fraction of natural fibers absorbs moisture, causing a detrimental effect on the dimensional stability and water resistance properties of any thermoplastic material to which the natural fibers are added.
As a result it is desirable to make use of the advantages linked to the use of organic materials of natural origin in creating composite materials, but by treating the natural fibers in a manner that improves the processing-relevant and use-relevant properties of the composite materials.